1. Field of the Invention
The present invention relates to a self-excited flyback switching power supply device for converting a high direct current voltage obtained by rectifying and smoothing a commercial power supply into a low direct current voltage necessary for a device, and to an image forming apparatus including the power supply device.
2. Description of the Related Art
In recent years, power saving is required in various electronic devices. Following the demands, further power saving is required also for a power supply of the electronic devices. As an example of the power supply of the electronic devices, there is used a switching-mode power supply (hereinafter referred to as “switching power supply”) for outputting a target voltage by driving a switching element such as a field effect transistor (FET) at a predetermined frequency. In some types of the switching power supply, the number of switching operations of the switching element is reduced in a power-saving operation (hereinafter referred to also as “light load operation”) to improve efficiency. The specifications for power saving have been subject to annual changes, and it has been required to improve efficiency by saving power in a normal operation as well as the light load operation.
Most of the losses of the switching power supply in the light load operation are caused by the switching operation. Therefore, the reduced number of the switching operations in the light load operation contributes greatly to a higher efficiency of the switching power supply. Thus, the following measure is often taken. In order to reduce the loss caused by the switching operation in the light load operation, the turn-ON time of the switching element is lengthened. This increases energy of each switching operation, thereby lengthening an inactive period of the switching operation to reduce the number of switchings per unit time.
The long inactive period of the switching operation, however, leads to a low switching frequency. As a result, operating sound of a transformer in the switching operation becomes audible to human ears. This is because mechanical vibration noise of the transformer driven in synchronization with the switching operation is shifted to the human audible frequency band (about 20 Hz to 20 kHz) due to the decrease in switching frequency.
One well-known method for reducing the vibration noise from the transformer is to suppress a magnetic field variation of the transformer to reduce the noise. For example, a method of using a core material having a large cross-sectional area for the transformer or a method of shortening the turn-ON time of the switching element to reduce a current of the transformer per switching has been employed in order to suppress the magnetic field variation of the transformer.
A known method for appropriately producing a driving current waveform of the transformer to alleviate the vibration noise of the transformer is to provide a soft-start circuit in the switching power supply device and to gradually change the duty cycle at the rising and falling edges of a voltage across a capacitor at the start of activation. By setting the driving current waveform of the transformer to be gradually larger or gradually smaller, the magnetic flux of the transformer does not change easily, and hence the generation of vibration noise can be reduced. Such a conventional method is described in, for example, Japanese Patent No. 3665984.
However, the above-mentioned methods of reducing the vibration noise of the transformer have the following problems. According to the method of using a core material having a large cross-sectional area for the transformer, the transformer is increased in size, and hence it is difficult to downsize the power supply device. According to the method of uniformly shortening the turn-ON time of the switching element, the turn-ON time is reduced to reduce the change in magnetic field of the transformer, and hence the vibration noise of the transformer is alleviated. However, the number of switchings per unit time increases, and the switching loss increases, resulting in control unfavorable to power saving. Further, the method of changing the driving current waveform of the transformer to be gradually larger or gradually smaller is difficult to be applied to the case of the light load operation where energy to be supplied to a load on the secondary side of the transformer is small. In other words, this method is difficult to be applied because the inactive period of the switching operation is provided in the light load operation and it is therefore difficult for the soft-start circuit to change the current waveform to be gradually larger or gradually smaller.
In the case of driving the transformer in the light load operation, the resonant frequency of the transformer needs to be taken into consideration. The sound pressure level of the vibration noise of the transformer generated in the light load operation varies depending on the driving frequency of the switching element. In particular, when the driving frequency of the switching element matches with the resonant frequency of the transformer, the sound pressure level becomes very high. The mechanical resonant frequency of the transformer used in a switching power supply depends on the shape of the core of the transformer, but has a peak of the resonant frequency (f0) at about several kHz to ten and several kHz. This frequency band is the audible band to humans, and is also a driving frequency band of the switching element that can operate in the light load operation. Therefore, if the switching element is driven at the resonant frequency of the transformer, a conspicuously recognizable harsh noise is generated from the transformer. In the above-mentioned conventional methods, the control of reducing the vibration noise is not performed in accordance with the characteristics of the transformer, and hence there has been a problem in that the vibration noise cannot be suppressed effectively in the switching operation in the light load operation.